Backlight unit and liquid crystal display unit using backlight unit

ABSTRACT

To compensate for uneven brightness in the longitudinal direction of fluorescent lamps of a backlight unit and achieve a display screen with an even brightness, a reflection unit of the backlight unit, the fluorescent tube surface of the fluorescent lamps or a diffusion unit is used to either reduce the reflectance, transmittance, or radiation brightness of the high-voltage side of the fluorescent lamps or increase the reflectance, transmittance, or radiation brightness of the low-voltage side thereof so as to compensate for uneven brightness of the illumination light and thereby ensure an even brightness. For example, dot pattern regions D 1 , D 2  and D 3 , i.e., the regions whose density increases in stages, are imparted to the portion of a reflection layer  13  of the backlight unit with a relatively high brightness. As for the display device, on the other hand, the display image data supplied to a liquid crystal panel or the aperture ratio of the liquid crystal panel is controlled, for example, to compensate for uneven brightness in the longitudinal direction of the fluorescent lamps and ensure an even brightness.

TECHNICAL FIELD

The present invention relates to a backlight unit operable to illuminatea target from the rear side and a liquid crystal display device usingthe backlight unit.

BACKGROUND OF THE INVENTION

A backlight unit is used to illuminate the target such as an LCD displaypanel. An LCD display device employs either one of two types ofbacklight configurations as a backlight unit; the direct type and theedge light type (light guide plate type).

With the direct type, fluorescent tubes, i.e., a light source, arearranged directly below the liquid crystal panel to be illuminated. Thisallows fluorescent tubes to be increased with the change in the displayscreen size, thus achieving a sufficient brightness. In this case,however, the backlight unit is prone to an uneven brightness betweenareas having a fluorescent lamp and others not. Moreover, the directtype backlight unit must be built with sufficient strength. For example,the backlight case is fabricated with a metal plate. Then, a reflectivesheet is affixed to the inner surface of the backlight, with a pluralityof straight tube lamps arranged thereabove.

With the edge light type, on the other hand, a fluorescent lamp isarranged at the edge of a light guiding body made, for example, of aclear acrylic plate. This type of backlight unit takes advantage ofmultireflection in the light guiding body to use one surface thereof asan area light source. The edge light type has a reflector at the back ofthe straight tube lamp and L-shaped lamp. Although the display deviceusing the edge light type backlight unit can be reduced in thickness,the light guiding body of the large-size model becomes excessivelyheavy. Besides, upsizing of the device makes it difficult to securesufficient screen brightness.

The aforementioned features are the reasons why, in general, the directtype backlight unit is used for a large-screen liquid crystal displaydevice, whereas the edge light type backlight unit is used for thosewith a small screen.

The fluorescent lamps used for the backlight unit as described above aredriven by a high voltage of 1 KV at a high frequency of 50 to 70 KHz toachieve even and high brightness. At this time, the fluorescent lampsdevelop uneven brightness, i.e., uneven brightness, in the form of abrightness gradient between the high- and low-voltage sides as a resultof a leak current. This problem is caused by the following reason. Thefluorescent lamps are driven by a high voltage at a high frequency. Thiscauses the air layer to act as a stray capacitance and leads to a leakcurrent flowing from the fluorescent lamps to the lamp reflector and thesurrounding metal objects. As a result, the current flowing into thelow-voltage side of the fluorescent lamps diminishes. This causes thelow-voltage side to illuminate relatively less brighter than thehigh-voltage side.

Therefore, if the fluorescent lamps are long, the leak current risesproportionally to the length thereof. In the presence of a large leakcurrent, the farther the fluorescent lamps are from the drive circuit,the darker they become. This constitutes the cause of uneven brightness.That is, the larger the liquid crystal display device, the more likelythe difference in brightness occurs between the high- and low-voltagesides of the lamps. It can be said that the technique allowing therealization of a backlight unit with minimal uneven brightness isessential.

FIG. 18 is an explanatory view of the brightness characteristic offluorescent lamps, illustrating an example of the brightnessdistribution in the longitudinal direction (i.e., in the direction ofvoltage application) of the fluorescent lamps generally used for abacklight type liquid crystal display device.

As shown in FIG. 18, the fluorescent lamps have a brightness gradientwhose relative brightness diminishes from a high-voltage side H to alow-voltage side L. The brightness drop is particularly noticeable nearthe edge of the low-voltage side L. The brightness distribution curveitself also varies depending on the shape of the fluorescent lamps, thelength of the fluorescent tube, the drive voltage or the drivefrequency. Basically, however, the fluorescent lamps develop unevenbrightness in the form of relatively low brightness at the low-voltageside L as compared with the high-voltage side H.

FIG. 19 is a graph showing the brightness distribution characteristic inthe longitudinal direction (in the direction of voltage application) ofthe fluorescent lamps having the brightness gradient shown in FIG. 18when the drive voltage is further raised. In the example of FIG. 19, thebrightness of the fluorescent lamps at the center and low-voltage side Lis roughly equal. However, the brightness is relatively higher near theedge at the high voltage side H. For example, assuming that thebrightness is 100 at the center and the low-voltage side, the brightnessis relatively higher or 115 to 125 at the high-voltage side H. Thebrightness, highest at the edge of the high-voltage side H, graduallydeclines toward the center of the fluorescent lamps.

The display screen also develops uneven brightness due to unevenbrightness developed by the fluorescent lamps in the longitudinaldirection as described above. As a technique to reduce such unevenbrightness in the display screen, the liquid crystal display deviceusing a backlight is known as shown below.

FIGS. 20A and 20B are explanatory views of an example of the liquidcrystal display device having a conventional direct type backlight unit.FIG. 20A illustrates a side cross-sectional schematic configuration ofthe LCD device, whereas FIG. 20B illustrates a plan schematicconfiguration of the fluorescent lamps, i.e., the light source of thebacklight unit.

As shown in FIGS. 20A and 20B, the backlight unit has a plurality offluorescent lamps 101, reflectors 102 adapted to reflect the light fromthe fluorescent lamps 101 and an optical diffusion unit 103 provided atthe front of the fluorescent lamps 101 and adapted to diffuse the lightdirectly incident from the fluorescent lamps 101 or that reflected bythe reflectors 102. The backlight unit is used to illuminate a liquidcrystal panel 104 provided at the front (surface side) thereof throughthe optical diffusion unit 103.

With the aforementioned backlight unit, the fluorescent lamps 101 arearranged in sets of two such that the high-voltage side of one lamp isadjacent to the low-voltage side of the other to compensate for unevenbrightness in the lamps 101 and achieve a display screen with evenbrightness.

That is, as shown in FIGS. 20A and 20B, the backlight unit is providedwith a plurality of sets (S₁, S₂, S₃ and beyond) of the two fluorescentlamps 101, with the high-voltage side H of one lamp adjacent to thelow-voltage side L of the other lamp. Such a configuration cancels outuneven brightness resulting from each fluorescent lamp, thus eliminatinguneven brightness on the display screen and achieving an even display.

A liquid crystal display device in Patent Document 1 is disclosed as anexample with the high- and low-voltage sides H and L arranged adjacentto each other.

Further, the technique as shown in FIGS. 21A and 21B is available thatis associated with the liquid crystal display device operable to improvethe reflectance of the light from the backlight. FIGS. 21A and 21Billustrate another example of the backlight unit in a conventionalliquid crystal display device. FIG. 21A illustrates a sidecross-sectional schematic configuration of the backlight unit, whereasFIG. 21B illustrates a plan schematic configuration of the inside of theunit, with the optical diffusion sheet, provided on the backlight unitsurface, removed. In FIGS. 21A and 21B, reference numeral 201 denoteslinear fluorescent lamps, 202 optical diffusion sheets, 203 a reflectionsheet and 204 a reflection layer and 205 an enclosure.

The backlight unit shown in FIGS. 21A and 21B has the reflection layer204, made of a high reflectance material such as aluminum, on the innersurface at the bottom of the enclosure 205 further at the back of thereflection sheet 203 provided at the back of the linear fluorescentlamps 201 to efficiently enhance the brightness. Here, of the lightincident on the reflection sheet 203, the fraction that passes throughthe sheet 203 without being reflected is reflected again by thereflection layer 204 back toward the reflection sheet 203, rather thandisappears or becomes diffused at the back of the reflection sheet 203.This ensures efficient use of the light passing through the sheet 203from the back, thus enhancing the brightness.

In general, a foamed PET (Poly Ethylene Terephthalate) sheet is oftenused for the direct type reflection unit (equivalent to the reflectionsheet 203 described above). The foamed PET reflection sheet ismanufactured by foaming PET to produce fine air bubbles within thesheet. The light incident on the foamed PET sheet is refracted by theair bubbles to regress and emerge again from the incident side. Such alight reflection takes advantage of the refraction characteristicbetween the PET material and the air in the air bubbles, thus minimizinglight loss and achieving a high reflectance reflection unit, despite theuse of an inexpensive member. In addition to the above, other materialsmay be alternatively used including those coated on the surface with ahigh reflectance material such as silver or aluminum.

For example, while the reflection sheet 203, formed with a foamed PETsheet as described above, achieves a high reflectance, part of theincident light from the light source passes through the foamed PET sheetto the rear side (back side opposite to the light source). This leads toreduced light utilization efficiency. To improve these points forenhanced light utilization efficiency, the reflection layer 204 made ofa high reflectance material such as aluminum is provided on the innersurface of the enclosure 205 at the back of the reflection sheet 203 toreflect the light passing through the reflection sheet 203 with thereflection layer 204. Part of the reflected light from the reflectionlayer 204 passes again through the reflection sheet 203 and emerges onthe front side (light source side). This ensures improved lightutilization efficiency.

An edge light type backlight device using a light guide plate isdisclosed, for example, in Patent Document 2 as the backlight devicehaving another reflection layer stacked at the back of the reflectionsheet as described above.

Further, Patent Document 3 discloses a technique that changes the leakcurrent flowing between the high- and low-voltage sides of thefluorescent tube in an edge light type backlight unit to suppress theuneven brightness of the screen. With this backlight unit, thefluorescent lamp is shaped to have straight tube portions in one piece;the one portion running along one of the longer sides of the light guideplate and the other portions each running along one of the shorter sidesof the plate. The reflector, provided on the straight tube portion atthe high-voltage side of the fluorescent lamp, i.e., one of the tubeportions running along the shorter sides of the light guide plate, isformed with a white reflecting member, whereas the reflector at thelow-voltage side is deposited on the inside with silver. Such aconfiguration changes the leak current flowing between the high- andlow-voltage sides, thus securing a proper fluorescent lamp length togenerate necessary brightness over the rectangular screen and minimizingthe difference in brightness between the left and right sides of thescreen.

Further, the problem here derives from the driving at a high frequency.Therefore, the method is under consideration to drive the fluorescentlamp at the lowest possible frequency for increased the impedance of thestray capacitance and reduced leak current, thus eliminating unevenbrightness.

Description will be given next of the problems associated with theconventional techniques described above.

With the liquid crystal display device described in Patent Document 1,the fluorescent lamps are arranged parallel with each other in sets oftwo such that the high-voltage side H of one lamp is adjacent to thelow-voltage side L of the other. At this time, because of the proximitybetween the high-voltage side terminal of one fluorescent lamp and thelow-voltage side terminal of the other lamp adjacent thereto, dischargemay occur between the two electrodes. This renders the stable dischargeof the fluorescent lamps itself extremely difficult and possiblydeteriorates the reliability of the device.

Moreover, the high- and low-voltage terminals of the fluorescent lampsare disposed separately on both sides of the display screen. Thisrequires two inverter power circuits, resulting in higher cost. Further,the thinner and larger the display device, the more difficult it is tomake wiring connections to the fluorescent lamps. As a result,additional measures are required to ensure wiring safety and prevent thecurrent leak.

With the backlight device of Patent Document 2, on the other hand, ifthe brightness distribution of the fluorescent lamp is not uniform inthe longitudinal direction, the entire display screen may develop unevenbrightness as a result of the uneven brightness of the fluorescent lamp.This makes it difficult to control the brightness distribution. Inparticular, the GND side (low-voltage side) is prone to current leakfrom the fluorescent lamp. This results in high brightness only at thehigh-voltage side of the fluorescent lamp and low brightness at the GNDside.

In the case of Patent Document 3, provision of the white reflector onlyon one of the shorter sides of the fluorescent lamp alone cannotcompensate for the brightness gradient inherently present in thefluorescent lamps. The fluorescent lamp invariably develops a brightnessgradient at least along its longer sides. This results in unevenbrightness in the liquid crystal display device. If the fluorescent lampis longer as a result of the upsizing of the liquid crystal displaydevice, the aforementioned problem becomes more noticeable.

Further, while the method of lighting the lamps at a lower drivefrequency could be possible to the extent that thermal runaway does notoccur in the transformer, an excessively low frequency design coulddegrade the reliability. Besides, lowering the drive frequency willresult in larger components such as the transformer.

In light of the foregoing, the present invention was conceived and theobject thereof is to provide a backlight unit operable to compensate forthe brightness difference between the high- and low-voltage sides of thefluorescent lamps, provided as a light source, and to ensure an evenbrightness of the outgoing light, and a liquid crystal display deviceoperable to ensure an even brightness over the entire display screen.

Patent Document 1: Japanese Laid-Open Patent Publication No. H11-295731Patent Document 2: Japanese Laid-Open Patent Publication No. H08-335048Patent Document 3: Japanese Laid-Open Patent Publication No. H10-112213

DISCLOSURE OF THE INVENTION

The first technical measure of the present invention is characterized bya backlight unit operable to illuminate the target with fluorescentlamps, the backlight unit comprising brightness compensation meansadapted to compensate for uneven brightness in the longitudinaldirection of the fluorescent lamps.

The second technical measure of the present compensate for unevenbrightness in the longitudinal direction of the fluorescent lamps.

The third technical measure of the present invention is characterized bythe backlight unit of the second technical measure, wherein thebrightness compensation means have regions with relatively high and lowreflectances in the reflection unit and take advantage of the differencein reflectance to compensate for uneven brightness in the longitudinaldirection of the fluorescent lamps.

The fourth technical measure of the present invention is characterizedby the backlight unit of the third technical measure, wherein thebrightness compensation means have a reflectance gradient that causesthe reflectance of the reflection unit to decline gradually or in stagesand take advantage of the reflectance gradient to reduce the brightnessof the portion with a relatively high brightness in the longitudinaldirection of the fluorescent lamps.

The fifth technical measure of the present invention is characterized bythe backlight unit of the third or fourth technical measure, wherein thebrightness compensation means have a reflectance gradient that causesthe reflectance of the reflection unit to increase gradually or instages and take advantage of the reflectance gradient to increase thebrightness of the portion with a relatively low brightness in thelongitudinal direction of the fluorescent lamps.

The sixth technical measure of the present invention is characterized bythe backlight unit of any one of the second to fifth technical measures,wherein the brightness compensation means are a dot pattern provided onthe reflection unit and take advantage of the dot pattern to control thereflectance of the reflection unit.

The seventh technical measure of the present invention is characterizedby the backlight unit of the sixth technical measure, wherein thereflectance of the reflection unit provided with the dot pattern iscontrolled by one or a plurality of the reflectance of the group ofsmall dots making up the dot pattern, the dot density, the dot shape,and the dot color.

The eighth technical measure of the present invention is characterizedby the backlight unit of the first technical measure, comprising areflection unit adapted to emit the light from the fluorescent lamps ina specific direction, wherein the reflection unit is made up of firstand second reflection layers having given optical reflectance andtransmittance, wherein the reflection unit is configured with a firstregion having the first and second reflection layers stacked one aboveanother in the direction of incidence of light and a second region madeup only of the first reflection layer, and wherein the reflectance ofthe reflection unit is controlled using the first region with arelatively high reflectance and the second region with a reflectancelower than that of the first region.

The ninth technical measure of the present invention is characterized bythe backlight unit of the first technical measure, wherein thebrightness, compensation means are provided on a glass tube of thefluorescent lamps and control the transmittance of the glass tube tocompensate for uneven brightness in the longitudinal direction of thefluorescent lamps.

The tenth technical measure of the present invention is characterized bythe backlight unit of the first technical measure, comprising adiffusion unit adapted to diffuse the light from the fluorescent lamps,wherein the brightness compensation means are provided on the diffusionunit and control the transmittance of the diffusion unit to compensatefor uneven brightness in the longitudinal direction of the fluorescentlamps.

The eleventh technical measure of the present invention is characterizedby the backlight unit of the ninth or tenth technical measure, whereinthe brightness compensation means have regions with relatively high andlow transmittances in the glass tube or diffusion unit and takeadvantage of the difference in the transmittance to compensate foruneven brightness in the longitudinal direction of the fluorescentlamps.

The twelfth technical measure of the present invention is characterizedby the backlight unit of the eleventh technical measure, wherein thebrightness compensation means have a transmittance gradient that causesthe transmittance to decline gradually or in stages and take advantageof the transmittance gradient to reduce the brightness of the portionwith a relatively high brightness in the longitudinal direction of thefluorescent lamps.

The thirteenth technical measure of the present invention ischaracterized by the backlight unit of the eleventh or twelfth technicalmeasure, wherein the brightness compensation means have a transmittancegradient that causes the transmittance to increase gradually or instages and take advantage of the transmittance gradient to increase thebrightness of the portion with a relatively low brightness in thelongitudinal direction of the fluorescent lamps.

The fourteenth technical measure of the present invention ischaracterized by the backlight unit of any one of the ninth tothirteenth technical measures, wherein the brightness compensation meansare a dot pattern provided on the glass tube of the fluorescent lamps orthe diffusion unit and take advantage of the dot pattern to control thetransmittance.

The fifteenth technical measure of the present invention ischaracterized by the backlight unit of the fourteenth technical measure,wherein the transmittance of the glass tube or the diffusion unitprovided with the dot pattern is controlled by one or a plurality of thereflectance of the group of small dots making up the dot pattern, thedot density, the dot shape, and the dot color.

The sixteenth technical measure of the present invention ischaracterized by the backlight unit of the first technical measure,wherein the brightness compensation means are provided on the glass tubeof the fluorescent lamps and control the tube surface brightness of theglass tube to compensate for uneven brightness in the longitudinaldirection of the fluorescent lamps.

The seventeenth technical measure of the present invention ischaracterized by the backlight unit of the sixteenth technical measure,wherein the thickness of the fluorescent substance formed inside theglass tube of the fluorescent lamps as the brightness compensation meansis changed correspondingly with the longitudinal position of thefluorescent lamps to compensate for uneven brightness in thelongitudinal direction of the fluorescent lamps.

The eighteenth technical measure of the present invention ischaracterized by a liquid crystal display device comprising thebacklight unit of any one of the first to seventeenth technical measureand a liquid crystal panel illuminated by the backlight unit.

The nineteenth technical measure of the present invention ischaracterized by a liquid crystal display device operable to apply anillumination light from a backlight unit having fluorescent lamps to aliquid crystal panel to display images, the liquid crystal displaydevice comprising brightness compensation means adapted to compensatefor uneven brightness in the longitudinal direction of the fluorescentlamps.

The twentieth technical measure of the present invention ischaracterized by the liquid crystal display device of nineteenthtechnical measure, wherein the brightness compensation means have agradation conversion unit operable to carry out a given gradationconversion process of input image data and a control portion operable toswitch between gradation conversion characteristics of the gradationconversion unit based on a synchronizing signal of the input image data,and wherein the control portion switches from one gradation conversioncharacteristic to another in the gradation conversion unit based on thescreen position to display the image data to compensate for unevenbrightness in the longitudinal direction of the fluorescent lamps.

The twenty-first technical measure of the present invention ischaracterized by the liquid crystal display device of the nineteenthtechnical measure, wherein the liquid crystal panel is configured tohave, as the brightness compensation means, an aperture ratio thatchanges correspondingly with the display screen position, and whereinthe aperture ratio is changed to compensate for uneven brightness in thelongitudinal direction of the fluorescent lamps.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are explanatory views of an embodiment of a backlightunit according to the present invention;

FIG. 2 is an explanatory view of a layout example of fluorescent lampsin a backlight unit applied to the present invention;

FIG. 3 is an explanatory view of an example of a dot pattern imparted toa reflection layer;

FIGS. 4A and 4B are expanded views of the dot pattern of the reflectionlayer shown in FIG. 3;

FIG. 5 is an explanatory view of another embodiment of the backlightunit according to the present invention;

FIG. 6 is an explanatory view of still another embodiment of thebacklight unit according to the present invention;

FIGS. 7A and 7B are explanatory views of still another embodiment of thebacklight unit according to the present invention;

FIG. 8 is an explanatory view of still another embodiment of thebacklight unit according to the present invention;

FIGS. 9A to 9D are explanatory views of still another embodiment of thebacklight unit according to the present invention;

FIG. 10 illustrates an example of the relationship between the filmthickness of a fluorescent substance and the tube surface brightness atthat time;

FIG. 11 is an explanatory view of still another embodiment of thebacklight unit according to the present invention;

FIGS. 12A and 12B illustrate a configuration example of the edge lighttype backlight unit according to the present invention;

FIG. 13 is an explanatory view of an embodiment of a liquid crystaldisplay device according to the present invention;

FIG. 14 is a block diagram of the major components illustrating aschematic configuration of another embodiment of the liquid crystaldisplay device according to the present invention;

FIG. 15 is an explanatory view of the display screen region of theliquid crystal display device of FIG. 14;

FIG. 16 illustrates gradation conversion characteristics (input/outputcharacteristics) of a gradation conversion unit in the liquid crystaldisplay device of FIG. 14;

FIG. 17 is an explanatory view of the aperture ratio control in a liquidcrystal panel;

FIG. 18 is an explanatory view of an example of the relative brightnessdistribution characteristic in the longitudinal direction (in thedirection of voltage application) of the fluorescent lamps;

FIG. 19 is a graph illustrating the relative brightness characteristicin the longitudinal direction (in the direction of voltage application)of the fluorescent lamps when a drive voltage, applied to thefluorescent lamps having the brightness gradient shown in FIG. 18, isfurther raised;

FIGS. 20A and 20B are explanatory views of an example of the liquidcrystal display device having a conventional direct type backlight unit;and

FIGS. 21A and 21B illustrate another example of the backlight unit inthe conventional liquid crystal display device.

PREFERRED EMBODIMENT OF THE INVENTION

As described above, the fluorescent lamps of the backlight unit developa non-uniform brightness (uneven brightness) due to the relativelyhigher brightness at the high-voltage side. The present inventionimparts, to the backlight unit or the liquid crystal display device,brightness compensation means operable to compensate for the unevenbrightness in the longitudinal direction of the fluorescent lamps so asto compensate for the uneven brightness inherently present in the lampsand achieve a display screen with an even brightness.

To ensure an even brightness, the brightness compensation means impartedto the backlight unit provide:

-   (1) the reflection means intended to reflect the light of the    fluorescent lamps and direct it in a single direction with means to    reduce the reflectance of the high brightness portion of the    fluorescent lamps (high-voltage side) or increase the reflectance of    the low brightness portion of the fluorescent lamps (low-voltage    side);-   (2) the glass tube surface of the fluorescent lamps with means to    reduce the transmittance of the high brightness portion of the    fluorescent lamps (high-voltage side) or increase the transmittance    of the low brightness portion of the fluorescent lamps (low-voltage    side);-   (3) the glass tube surface of the fluorescent lamps with means to    reduce the radiation brightness of the high-voltage side of the    fluorescent lamps or increase the radiation brightness of the    low-voltage side of the fluorescent lamps; or-   (4) the diffusion sheet with means to reduce the transmittance of    the high brightness portion of the fluorescent lamps (high-voltage    side) or increase the transmittance of the low brightness portion of    the fluorescent lamps (low-voltage side). The above means may be    used in combination to ensure an even brightness.

To ensure an even brightness, on the other hand, the brightnesscompensation means imparted to the display device control:

-   (1) the image data supplied to the liquid crystal panel to    compensate for the uneven brightness in the longitudinal direction    of the fluorescent lamps; or-   (2) the aperture ratio of the liquid crystal panel to compensate for    the uneven brightness in the longitudinal direction of the    fluorescent lamps.

Description will be given below of the embodiments of the presentinvention that can accomplish the aforementioned brightness compensationmeans. It is to be noted that the same reference numerals are used todenote the elements, components or portions with similar functionsthroughout the drawings for describing the embodiments, and duplicateddescription is omitted.

Embodiment 1

In the present embodiment, the reflection layer in the backlight unit isprovided with the brightness compensation means adapted to compensatefor the brightness in the longitudinal direction of the fluorescentlamps so as to compensate for the uneven brightness of such lamps andevenly illuminate the target such as the liquid crystal display device.The brightness compensation means in the present embodiment are designedto control the reflectance of the light from the fluorescent lamps.

FIGS. 1A and 1B are explanatory views of an embodiment of the directtype backlight unit according to the present invention. FIG. 1A is aplan schematic view illustrating the inside of the backlight unit,whereas FIG. 1B is a schematic configuration diagram of the backlightunit taken along cross-section line B-B in FIG. 1A. In FIGS. 1A and 1B,reference numeral 10 denotes a backlight unit, 11 fluorescent lamps, 12an enclosure, 13 a reflection layer provided at the bottom of theenclosure, 14 a diffusion unit and 15 lamp supporting members. It is tobe noted that FIG. 1A illustrates the inside of the unit with thediffusion unit 14 shown in FIG. 1B removed.

The backlight unit 10 has a reflection unit adapted to emit the lightfrom the fluorescent lamps 11 in a specific direction. In the presentembodiment, the reflection layer 13 is provided as the reflection uniton the inner surface at the bottom of the enclosure 12 of the backlightunit 10. The enclosure 12 may be configured with a shielding plateadapted to shield electromagnetic waves generated from the fluorescentlamps 11.

The reflection layer 13 is held above the inner surface at the bottom ofthe enclosure 12 of the backlight unit 10 with a space therebetween ordirectly on the inner surface. A foamed PET sheet or a material with anoptical reflection surface made of silver or aluminum may be used, forexample, for this layer. As a foamed PET sheet, E60L or E60V type ofLumirror (R) from Toray may be preferably used.

The diffusion unit 14, provided at the front (surface) of thefluorescent lamps 11, is configured with a material having an opticaldiffusion characteristic such as acrylic plate to diffuse the incidentlight directly from the fluorescent lamps 11 or the light that isreflected by the reflection layer 13 and guided again back toward thefront. In addition to the above, a functional film or sheet such as areflective polarizing film, prism sheet or ITO sheet may be includedbetween the diffusion unit 14 and the fluorescent lamps 11 for use inthe liquid crystal display device.

The transmitted light passing through the diffusion unit 14 is used toilluminate the target (not shown) such as the liquid crystal panelprovided further at the front of the diffusion unit. To light theplurality of the fluorescent lamps 11, a high voltage is applied to thelamps 11 from an inverter power circuit (not shown).

FIG. 2 is an explanatory view of the layout of the fluorescent lamps 11,schematically illustrating a plan layout of the lamps. Here, theplurality of the fluorescent lamps 11 are laid out so as to belongitudinally parallel to each other. The high- and low-voltage sides Hand L of the fluorescent lamps 11 are arranged at the same sides so thatthe high-voltage side H of each of the lamps 11 is adjacent to that ofanother lamp and that the low-voltage side L of each of the lamps isadjacent to that of another lamp.

The fluorescent lamps 11 have an uneven brightness that causes thebrightness of the high-voltage side to be relatively higher in thelongitudinal relative brightness distribution as described above. In thepresent embodiment, the reflection layer 13 is provided with thebrightness compensation means tailored for the uneven brightness of thefluorescent lamps 11 to compensate for the uneven brightness in thelongitudinal direction inherently present in the lamps 11 and achieve adisplay screen with an even brightness.

As such brightness compensation means, two possible means are available.The first is to reduce the reflectance of the reflection layer 13 at theportion of the fluorescent lamps 11 that is relatively high inbrightness (high-voltage side H). The second is to increase thereflectance of the reflection layer 13 at the portion of the fluorescentlamps 11 that is relatively low in brightness (low-voltage side L).These two means may also be used in combination.

As an example of the brightness compensation means, a dot pattern isimparted to the reflection layer 13 to control the reflectance. The dotpattern controls the reflectance of the outgoing light from thefluorescent lamps 11, thus compensating for the uneven brightnessdeveloped by the fluorescent lamps 11 in the longitudinal direction.

FIG. 3 is an explanatory view of an example of the dot pattern impartedto the reflection layer 13. On the other hand, FIGS. 4A and 4B areexpanded views of the dot pattern of the reflection layer shown in FIG.3. FIG. 4A is an expanded view of a region D₃ in FIG. 3, whereas FIG. 4Bis an expanded view of a region D₁ in FIG. 3.

In the present embodiment, the dot pattern imparted to the reflectionlayer 13 reduces the reflectance of the layer 13. The reflectance of thematerial making up the dot pattern is relatively lower than that of thesurface of the reflection layer.

In the present embodiment, the reflection layer 13 is provided, as shownin FIG. 3, with regions D₁, D₂ and D₃, i.e., the regions whosereflectance decreases in stages from the low-voltage side L to thehigh-voltage side H of the fluorescent lamps 11. These regions areformed so as to correspond to the uneven brightness of the fluorescentlamps 11 and compensate for the uneven brightness of the fluorescentlamps 11. In the present embodiment, the dot pattern, equivalent to thebrightness distribution compensation in FIG. 18, is imparted to thereflection layer 13.

The dot pattern imparted to the reflection layer 13 in the presentembodiment has a dot density increasing in stages from the low-voltageside L to the high-voltage side H of the fluorescent lamps 11 in theregions D₁, D₂ and D₃ with the dot pattern so as to reduce thereflectance from the low-voltage side L to the high-voltage side H. Asshown in FIGS. 4A and 4B, for example, the dots of the dot pattern areequally sized, and the dot pattern closer to the high-voltage side H hasa higher dot density. This changes the reflectance of the reflectionlayer 13 correspondingly with the uneven brightness in the longitudinaldirection of the fluorescent lamps 11, thus achieving an illuminatinglight with an even brightness distribution.

To control the reflectance of the reflection layer with a dot pattern,while the reflectance of the reflection layer 13 can be controlled byimparting to this layer a dot pattern to reduce the reflectance of thereflection surface of the reflection layer 13 as described above, thereflectance of the reflection layer 13 may be controlled by imparting tothis layer a dot pattern to increase the reflectance of the reflectionsurface of the reflection layer 13. In this case, a dot pattern adaptedto relatively increase the reflectance is provided in the region of thereflection layer 13 with a relatively low brightness in terms of thebrightness distribution of the fluorescent lamps 11. For example, if afoamed PET sheet is used for the reflection layer 13, a dot pattern,made of a high reflectance material such as silver or aluminum, isimparted to the region of the reflection layer 13 corresponding to thelow brightness region of the fluorescent lamps 11. This allows tocompensate for the uneven brightness in the longitudinal direction ofthe fluorescent lamps 11.

On the other hand, the dot pattern adapted to control the reflectance asdescribed above, may not only vary the equally shaped dot density asshown in the example of FIGS. 3 and 4A and 4B but also the dot shape(size) to control the reflectance. Further, the dot shape and densitymay be used in combination. Further, taking advantage of the change inthe reflectance with change in the dot color, the dot color as well asthe dot shape and density may be used in combination to control thereflectance. For example, the dot shape of the dot pattern may becircular, triangular, polygonal, star-shaped or elliptical, whereas thedot color may be gray, dark brown, silver, green, black, white orpurple.

Further, the dot pattern may impart a gradient that gradually reducesthe reflectance from the low-voltage side L to the high-voltage side H(that is, gradually increases the reflectance from the high-voltage sideH to the low-voltage side L) correspondingly with the uneven brightnessof the fluorescent lamps 11, rather than changes the reflectance instages as shown in the example of FIG. 3. Such a reflectance gradientcan be realized when the dot shape, size, density and color are usedalone or in combination with each other.

Ink can be imparted to the reflection layer 13, for example, throughscreen or ink jet printing to form the dot pattern imparted to thereflection layer 13. In addition to printing, the dot pattern may beformed using other means, namely, sputtering, vapor deposition,photolithography, optical machining using a laser beam or lamination ofclear dot-patterned films.

As another specific example of the brightness compensation means, thereflection layer 13 can be coated with an ink or dye with varyingconcentration to reduce or increase the reflectance of the reflectionunit gradually or in stages. To change the concentration at this time,the concentration of the dye or pigment itself may be varied, or thethickness of the film applied may be varied to change the apparentconcentration.

Moreover, a plurality of materials with different reflectances may beimparted to the surface of the reflection layer 13 as the brightnesscompensation means to change the reflectance in stages. Further, thesurface roughness of the reflection layer 13 may be varied to controlthe reflectance based on the difference of the optical diffusion orabsorption characteristic of the surface.

Further, two different measures, one to relatively reduce thereflectance of the reflection layer 13 and the other to increase it asdescribed above, may be used in combination to control the reflectanceof the reflection layer 13.

Embodiment 2

FIG. 5 is an explanatory view of still another embodiment of thebacklight unit according to the present invention, illustrating aschematic cross-section corresponding to the cross-section along lineB-B of the backlight unit in FIG. 1A. The backlight unit of the presentembodiment has, as the reflection unit, a reflection surface 12 aadapted to reflect the light of the fluorescent lamps 11 toward thediffusion unit 14 in place of the reflection layer 13 in theembodiment 1. The reflection surface 12 a is formed with a reflectivefilm made of a high reflectance material such as silver or aluminum andprovided on the inner surface at the bottom of the enclosure 12. On theother hand, the high- and low-voltage sides H and L of the fluorescentlamps 11 are arranged at the same sides as shown in FIG. 2.

In the present embodiment, the brightness compensation means areprovided on the reflection surface 12 a to control the opticalreflectance as described in the embodiment 1. This compensates for thereflectance of the reflection surface 12 a correspondingly with thelongitudinal brightness distribution of the fluorescent lamps 11, thusachieving an illumination light with an even brightness distribution. Asfor a specific configuration of the brightness compensation means, thebrightness compensation means in the embodiment 1 can be used.Therefore, duplicated description is omitted.

Embodiment 3

FIG. 6 is an explanatory view of still another embodiment of thebacklight unit according to the present invention, illustrating aschematic cross-section corresponding to the cross-section along lineB-B of the backlight unit in FIG. 1A. The backlight unit of the presentembodiment has, as the reflection unit, the reflection layer 13 shown inthe configuration of FIGS. 1A and 1B and the reflection surface 12 ashown in FIG. 5. On the other hand, the high- and low-voltage sides Hand L of the fluorescent lamps 11 are arranged at the same sides asshown in FIG. 2.

The enclosure 12 of the backlight unit 10 is provided with thereflection layer 13 as described in embodiment 1. While the reflectionlayer 13, made, for example, of the foamed PET sheet, is capable ofreflecting the light from the fluorescent lamps 11, part of the lightpasses through the reflection layer 13 to emerge at the rear side. Thereflection surface 12 a as described in the embodiment 2 is provided onthe inner surface at the bottom of the backlight unit 10. This surfacereflects the light passing through the reflection layer 13 back towardthe reflection layer 13. The light reflected by the reflection surface12 a is separated again into reflected and transmitted lights at thereflection layer 13. The transmitted light travels toward the diffusionunit 14 so that it will be effectively used.

The reflection layer 13 is supported using a supporting body in theshape of a frame and a lamp holder or supporting members such as screwsor stays. The reflection layer 13 allows an air layer to mediate betweenthis layer 13 and the reflection surface 12 a without coming in closecontact with the reflection surface 12 a. To allow the mediation of theair layer, while a given gap may be provided between the reflectionlayer 13 and the reflection surface 12 a, it suffices to simply placeand support the reflection layer 13 on the reflection surface 12 a. Thatis, because of the presence of a thin air layer on the back surface ofthe reflection layer 13, the difference in refraction index between thereflection layer 13 and air becomes greater on the back surface of thereflection layer 13. This will enhance the reflectance of the reflectionlayer 13. For example, if a material such as an adhesive having arefraction index close to that of the reflection layer 13 is provided onthe back surface of this layer 13, the component of transmitted lightincreases at the reflection layer 13. This will impair the opticalreflection characteristic.

While, in the present embodiment, the brightness compensation meansdescribed in the aforementioned embodiment may be imparted to thereflection layer 13 to achieve an even illumination light, the abovemeans may be further imparted to both of the reflection layer 13 and thereflection surface 12 a or only to the reflection surface 12 a. Thebrightness compensation means imparted to the reflection surface 12 acontributes only to transmitted light passing through the reflectionlayer 13. Therefore, the reflectance distribution must be designed basedon the reflectance (that is, transmittance) of the reflection layer 13.

Embodiment 4

FIGS. 7A and 7B are explanatory views of still another embodiment of thebacklight unit according to the present invention. FIG. 7A is a planschematic view illustrating the inside of the backlight unit, whereasFIG. 7B illustrates a configuration, including the diffusion unit 14, onthe cross-section along the fluorescent lamps 11 in FIG. 7A. In FIGS. 7Aand 7B, reference numeral 16 denotes screws used as the holding means ofreflection layers 13 a and 13 b. On the other hand, the high- andlow-voltage sides H and L of the fluorescent lamps 11 are arranged atthe same sides as shown in FIG. 2.

The backlight unit of FIGS. 7A and 7B is provided with the tworeflection layers 13 a and 13 b to emit the light from the fluorescentlamps in a specific direction. The reflection layers 13 a and 13 b eachhave a characteristic similar to the aforementioned foamed PET sheet,thus reflecting light at a high reflectance. However, part of theincident light passes through the reflection layers to the rear side. Inthe present embodiment, two regions are provided, a region W with thereflection layers 13 a and 13 b stacked one above another vertically (inthe direction of incidence of light) and a region S with only thereflection layer 13 b.

As described above, part of the incident light passes through thereflection layers 13 a and 13 b to the rear side. In the region W withthe reflection layers 13 a and 13 b stacked one above another, thetransmitted light passing through the reflection layer 13 a, i.e., thefirst layer provided on the front side (side of the fluorescent lamps11), is reflected by the reflection layer 13 b, i.e., the second layerprovided on the rear side, back to the first reflection layer 13 a. Thetransmitted light passing through the first reflection layer 13 atravels toward the diffusion unit 14 so that it will be effectivelyused.

In the region S with only the second reflection layer 13 b, on the otherhand, although the light reflected by this layer 13 b is effectivelyused, the transmitted light passing through the reflection layer 13 bdisappears or becomes diffused at the rear side thereof. Even if thetransmitted light returns to the reflection layer 13 b as a result ofreflection, for example, by the inner surface of the enclosure 12, onlya small percentage of such a light will be effectively used. Therefore,when the regions W and S are compared, the region W with the two stackedreflection layers 13 a and 13 b achieves a relatively higher reflectancethan the region S with only the reflection layer 13 b.

It is to be noted that although the second reflection layer 13 b on therear side is larger than the first reflection layer 13 a on the frontside to form the regions W and S in the configuration of FIGS. 7A and7B, the first reflection layer 13 a may be larger.

Using the two reflection layers 13 a and 13 b, the region W having theselayers is provided at the low brightness area of the fluorescent lamps11. This ensures a relatively higher reflectance, thus achieving anillumination light with an even brightness distribution.

A half mirror may be used, for example, for the first reflection layer13 a. Using the half mirror enhances the transmittance of the lightreflected by the second reflection layer 13 b back to the firstreflection layer 13 a (half mirror), thus achieving a high reflectance.

On the other hand, the brightness compensation means as described in theembodiments 1 to 3 may be used in combination with the configurationhaving the two reflection layers 13 a and 13 b.

When the two regions, i.e., the region W with the reflection layers 13 aand 13 b stacked one over another and the region S with only thereflection layer 13 b, are formed as in the present embodiment, holdingmembers are preferably provided on each of the reflection layers 13 aand 13 b, and particularly, on the layer 13 a on the front side toimpart a holding stability. For example, through-holes are made in allof the members, i.e., the enclosure 12 and the first and secondreflection layers 13 a and 13 b, as shown in FIGS. 7A and 7B. Then, thescrews 16 are inserted into the through-holes to hold the reflectionlayers 13 a and 13 b with the inner surface of the enclosure 12. Thissuppresses the bending of the reflection layers 13 a and 13 b due to thegravity and the like, thus maintaining their shapes. It is to be notedthat not only screws but also publicly known means that can holdreflection layers 13 a and 13 b with the inner surface of the enclosurecan be used as the holding means.

It is to be noted that, to prevent the holding means such as the screws16 from showing up on the display screen, the holding means arepreferably arranged so as to be hidden behind the fluorescent lamps 11as shown in FIG. 7B. Further, the holding means may be provided with acapability to hold the reflection layers 13 a and 13 b and another tohold the fluorescent lamps 11.

Embodiment 5

FIG. 8 is an explanatory view of still another embodiment of thebacklight unit according to the present invention, illustrating thefluorescent lamp 11 in a plan schematic view. In the present embodiment,the glass tube making up the fluorescent lamp 11 is provided with thebrightness compensation means to compensate for the uneven brightness ofthe lamp 11 and achieve a light with an even brightness distribution.Here, the brightness compensation means provided on the glass tube areused to control the optical transmittance of the glass tube of thefluorescent lamp 11, rather than control the reflectance as described inthe aforementioned embodiments. However, both means share the sametechnical principle of controlling the amount of light emitted to thetarget to ensure an even brightness.

In FIG. 8, a dot pattern is used as the brightness compensation means toreduce the optical transmittance of the glass tube. Here, three dotpattern regions with different densities, namely, regions D₁₁, D₁₂ andD₁₃, are disposed such that the dot density increases in stages from thelow-voltage side L to the high-voltage side H of the fluorescent lamp11. The regions D₁₁, D₁₂ and D₁₃ are formed to correspond to andcompensate for the uneven brightness of the fluorescent lamp 11. In thepresent embodiment, the dot pattern, equivalent to the brightnessdistribution compensation in FIG. 19, is imparted to the glass tube ofthe fluorescent lamp 11.

With the dot pattern imparted to the reflection layer 13 in the presentembodiment, the dot density is increased in stages from the low-voltageside L to the high-voltage side H of the fluorescent lamp 11 in the dotpattern regions D₁₁, D₁₂ and D₁₃ so as to reduce the transmittance fromthe low-voltage side L to the high-voltage side H of the lamp 11. Asshown in FIG. 8, for example, the dots of the dot pattern are equallysized, and the dot density of the dot pattern is higher in the regioncloser to the high-voltage side H. This changes the transmittance of theglass tube correspondingly with the uneven brightness in thelongitudinal direction of the fluorescent lamp 11, thus achieving anillumination light with an even brightness distribution.

The dot pattern adapted to control the transmittance as described abovemay change not only the density of the equally-shaped dots as shown inFIG. 8 but also the dot shape (size). Further, the dot shape and densitymay be used in combination. Further, the dot color may be changed tochange the transmittance. For example, the dot shape of the dot patternmay be circular, triangular, polygonal, star-shaped or elliptical,whereas the dot color may be gray, dark brown, silver, green, black,white or purple.

Further, the dot pattern, as described above, may impart a gradient thatgradually reduces the transmittance from the low-voltage side L to thehigh-voltage side H in correspondence with the uneven brightness of thefluorescent lamp 11 without changing the transmittance in stages asshown in the example of FIG. 8. Such a transmittance gradient can berealized when the dot shape, size, density and color are used alone orin combination with each other.

Ink is imparted to the glass tube, for example, through screen or inkjet printing to form the dot pattern imparted to the glass tube surface.In addition to printing, the dot pattern may be formed using othermeans, namely, sputtering, vapor deposition, photolithography, opticalmachining using a laser beam or lamination of clear films with a dotpattern.

As another specific example of the brightness compensation meansimparted to the glass tube of the fluorescent lamp 11, the glass tubecan be coated with an ink or dye with varying concentration to reduce orincrease the transmittance in stages or gradually. To change theconcentration at this time, the concentration of the dye or pigmentitself may be varied, or the thickness of the film applied may be variedto change the apparent concentration.

Moreover, a plurality of materials with different transmittances may beimparted to the glass tube surface as the brightness compensation means.Further, the surface roughness of the glass tube may be varied tocontrol the transmittance based on the difference of the opticaldiffusion or absorption characteristic of the surface.

Embodiment 6

FIGS. 9A to 9D are explanatory views of still another embodiment of thebacklight unit according to the present invention. FIG. 9A is across-sectional schematic view of the backlight unit, whereas FIGS. 9Bto 9D are cross-sectional schematic views of the fluorescent lamp 11taken respectively along cross-section lines B-B, C-C and D-D in FIG.9A. In FIGS. 9A to 9D, reference numeral 11 a denotes a glass tubemaking up the fluorescent lamp, 11 b a fluorescent substance provided onthe inner surface of the glass tube and d a film thickness of thefluorescent substance.

In the present embodiment, the brightness compensation means adapted tocompensate for the uneven brightness of the fluorescent lamp 11 andachieve a light with an even brightness distribution change the filmthickness d of the fluorescent substance 11 b formed on the inside ofthe glass tube 11 a of the fluorescent lamp 11 in the longitudinaldirection of the fluorescent lamp 11 to compensate for the unevenbrightness of the fluorescent lamp 11 during lighting.

That is, the present embodiment takes advantage of the change in tubesurface brightness with variation in the film thickness d of thefluorescent substance 11 b to change the film thickness d of thefluorescent substance 11 b correspondingly with the longitudinalposition of the fluorescent lamp 11 and achieve an even radiationbrightness in the longitudinal direction of the fluorescent lamp 11. Inthe example of FIGS. 9A to 9D, the film thickness d of the fluorescentsubstance is optimal for maximum brightness at the low-voltage side L ofthe fluorescent lamp 11 with relatively low brightness, and the filmthickness d is increased toward the high-voltage side H with relativelyhigh brightness.

FIG. 10 illustrates an example of the relationship between the filmthickness d of the fluorescent substance and the tube surface brightness(radiation brightness) at that time. As shown in FIG. 10, the brightnessof the lamp during lighting generally changes correspondingly with thefilm thickness d of the fluorescent substance, regardless of what thefluorescent substance is made of. The optimal value for the filmthickness d exists that allows the fluorescent substance to emit thebrightest light. That is, as shown in FIG. 10, if the film thickness dis smaller than the optimal value, the light is darker due to the lackof the fluorescent substance, whereas if the film thickness d is greaterthan the optimal value, the light is also darker due to the scatteringof light in the film.

The present embodiment conversely takes advantage of the above-describedcharacteristic to change the film thickness d of the fluorescentsubstance 11 b from the low-voltage side L of the fluorescent lamp 11with relatively low brightness to the high-voltage side H thereof withhigher brightness. At this time, the brightness declines irrespective ofwhether the film thickness d is smaller or greater than the optimalvalue as described above. For example, therefore, the fluorescentsubstance 11 b at the low-voltage side L with lower brightness is set tothe optimal film thickness, and the film thickness d is reduced orincreased toward the high-voltage side H with relatively highbrightness.

It is to be noted that, as described in embodiments 5 and 6, the methodof imparting the brightness compensation means to the glass tube itselfof the fluorescent lamp 11 can be applied not only to a straight tubefluorescent lamp but also to U-shaped, block C-shaped and L-shapedfluorescent lamps.

Embodiment 7

FIG. 11 is an explanatory view of still another embodiment of thebacklight unit according to the present invention. In the presentembodiment, the diffusion unit 14 is provided with the brightnesscompensation means to compensate for the uneven brightness in thelongitudinal direction of the lamps and achieve a light with an evenbrightness distribution. A diffusion plate or sheet capable of diffusinglight is used for the diffusion unit 14. The brightness compensationmeans are provided on the surface of the diffusion unit 14 to controlthe optical transmittance.

For example, a dot pattern is imparted to the surface of the diffusionunit 14 as shown in FIG. 11 to reduce the optical transmittance. Here,three dot pattern regions with different densities, namely, regions D₂₁,D₂₂ and D₂₃, are disposed such that the dot density increases in stagesfrom the low-voltage side L to the high-voltage side H of thefluorescent lamps 11. The regions D₂₁, D₂₂ and D₂₃ are formed tocorrespond to and compensate for the uneven brightness in thelongitudinal direction of the fluorescent lamps 11. Here, the dotpattern is preferably provided on the back side (side of the fluorescentlamps 11) rather than on the front side (side opposite to thefluorescent lamps 11) of the diffusion unit 14 because of a smallerlikelihood of the dot pattern to hinder the even diffusioncharacteristic of the diffusion unit 14 across the surface.

In addition to the above, the aforementioned brightness compensationmeans provided on the fluorescent lamps 11 in the embodiment 5 may beused in the same manner as the brightness compensation means to controlthe optical transmittance as described above. In the present embodiment,on the other hand, the thickness of the diffusion unit 14 may be changedcorrespondingly with the uneven brightness in the longitudinal directionof the fluorescent lamps 11 to change the transmittance of the lightpassing through the diffusion unit 14 and compensate for the unevenbrightness of the fluorescent lamps 11.

Embodiment 8

While the configuration examples of the direct type backlight unit havebeen described in the above embodiments, the backlight unit according tothe present invention may be applied to the edge light type as well asto the direct type. That is, the brightness compensation means impartedto the reflection layer or surface making up the reflection unit, thefluorescent lamps and the diffusion unit are not specifically applicableto the direct type backlight units. Instead, the brightness compensationmeans can compensate for the uneven brightness in the longitudinaldirection of the fluorescent lamp in the edge light type backlight unitsto achieve an illumination light with an even brightness.

Description will be given of an example of the edge light type backlightunit. FIGS. 12A and 12B are explanatory views of a configuration exampleof the edge light type backlight unit according to the presentinvention. FIG. 12A is an explanatory view with some of the componentsremoved in a plan schematic view of the backlight unit, whereas FIG. 12Billustrates a cross-sectional configuration along the longitudinaldirection of the fluorescent lamps.

With the edge light type backlight unit 10 shown in FIGS. 12A and 12B,the fluorescent lamp 11 is arranged at the side of a light guide plate17 as an edge light. The reflection layer 13 is provided at the back ofthe light guide plate 17. The light of the fluorescent lamp 11 is guidedto the front by the light guide plate 17 and the reflection layer 13 andthen emitted from the surface of the diffusion unit 14 as anillumination light. The reflection layer 13, i.e., the layer equivalentto the reflection layer 13 in the first embodiment, can be formed with afoamed PET sheet or a material having a high reflectance reflectionsurface made, for example, of silver or aluminum.

Thus, the brightness compensation means described in the aboveembodiments are imparted to one or a plurality of the reflection layer13, the fluorescent lamp 11 and the diffusion unit 14 correspondinglywith the uneven brightness of the fluorescent lamp 11. This compensatesfor the uneven brightness in the longitudinal direction of thefluorescent lamp 11 in the edge light type backlight unit as well, thusachieving an illumination light with an even brightness. That is, as forthe fluorescent lamp 11, it suffices to provide the brightnesscompensation means to compensate for the uneven brightness from thehigh-voltage side H to the low-voltage side L of the lamp 11. As for thereflection layer 13 and the diffusion unit 14, on the other hand, itsuffices to impart the brightness compensation means as described abovecorrespondingly with the uneven brightness of the light emitting surfaceof the light guide plate 17 resulting from the uneven brightness fromthe high-voltage side H to the low-voltage side L of the fluorescentlamp 11.

Embodiment 9

When configured with a backlight unit having the brightness compensationmeans as shown in the above embodiments, the liquid crystal displaydevice offers a high-quality display image free of uneven brightness onthe display screen.

FIG. 13 is an explanatory view of an embodiment of the liquid crystaldisplay device according to the present invention, illustrating across-sectional schematic configuration of the liquid crystal displaydevice having a backlight unit. In FIG. 13, reference numeral 20 denotesa liquid crystal display device and 21 a liquid crystal panel.

The liquid crystal display device 20 has the ordinary liquid crystalpanel 21, mainly configured by a liquid crystal material sealed betweentwo clear insulating substrates, and the backlight unit 10 operable toapply light to the liquid crystal panel 21. The backlight unit accordingto one of the first to eighth embodiments can be used for the backlightunit 10 in the liquid crystal display device 20 according to the presentembodiment.

When the liquid crystal panel 21 is illuminated with the backlight unit10 provided with the brightness compensation means according to thepresent invention, the uneven brightness is compensated for in thelongitudinal direction of the fluorescent lamps 11. This achieves anillumination light with even brightness, thus achieving a high-qualitydisplay screen free of uneven brightness on the liquid crystal panel 21.

As described above, the liquid crystal display device 20 ensures a highlight utilization efficiency when a polarizing reflective film, that isnot shown, is provided between the liquid crystal panel 21 and thediffusion unit 14 of the backlight unit 10. Here, the polarizationtransmission axis of the polarizing reflective film is aligned with thatof the polarizer at the incident side of the liquid crystal panel 21.Then, if, as a result of the diffusion or reflection of the polarizationfraction reflected by the polarizing reflective film, for example, bythe diffusion unit 14 or the reflection layer 13, the polarizationfraction thereof in the orthogonal direction (fraction coinciding withthe polarization transmission axis) occurs, this fraction passes throughthe polarizing reflective film and therefore can be used as theeffective light to the liquid crystal panel 21. Thus, the polarizingreflective film can efficiently produce a uniformly polarizedillumination light. A liquid crystal display device with high lightutilization efficiency can be obtained when the polarization directionof this light coincides with the polarization axis of the polarizer atthe incident side of the liquid crystal panel. Further, a functionalfilm or sheet such as an ITO sheet, diffusion film or prism sheet may beprovided between the polarizing reflective film and the diffusion unit14.

Embodiment 10

The present embodiment controls the display image data supplied to theliquid crystal panel in the liquid crystal display device to compensatefor the uneven brightness in the longitudinal direction of thefluorescent lamp and achieve a display screen with an even brightness.The present embodiment will be described below with reference to FIGS.14 to 16. Here, FIG. 14 is a block diagram of the major componentsillustrating a schematic configuration of the liquid crystal displaydevice according to the present embodiment. FIG. 15 is an explanatoryview illustrating the display screen region in the liquid crystaldisplay device according to the present embodiment. FIG. 16 is anexplanatory view illustrating gradation conversion characteristics(input/output characteristics) of a gradation conversion unit in theliquid crystal display device according to the present embodiment.

As shown in FIG. 14, the liquid crystal display device according to thepresent embodiment is provided with a gradation conversion unit 31operable to carry out a given gradation conversion process of inputimage data and an LCD control portion 32 operable to output an LCD drivesignal to a gate driver 34 and a source driver 35 of a liquid crystalpanel 33 based on the image data whose gradation has been converted bythe gradation conversion unit 31. The liquid crystal display device isalso provided with a microcomputer 36 that not only switches between thegradation conversion characteristics of the gradation conversion unit 31based on a synchronizing signal of the input image data but alsocontrols a light source drive unit 38 to drive a backlight source(linear fluorescent lamps) 37.

That is, the microcomputer 36 determines, based on the synchronizingsignal of the input image data, the screen position to display the imagedata, and instructs the gradation conversion unit 31 to switch betweenthe gradation conversion characteristics of the gradation conversionunit 31 based on the screen position. Here, we assume that the displayscreen is divided into three regions, as shown in FIG. 15, a region D₃₁of the screen corresponding to the low-voltage side of the linearfluorescent lamps 37, a region D₃₂ of the screen corresponding to theslightly higher voltage side of the lamps 37 and a region D₃₃ of thescreen corresponding to the highest voltage side of the lamps 37, andthat the gradation conversion characteristic for the data is switcheddepending on which of the regions D₃₁ to D₃₃ is used to display thedata.

The gradation conversion unit 31 has three gradation conversioncharacteristics as shown in FIG. 16 that can be switched from one toanother, i.e., a gradation conversion characteristic a adapted to outputthe input gradation level as is (without converting it), a gradationconversion characteristic b adapted to output the gradation level afterslightly suppressing the input level and a gradation conversioncharacteristic c adapted to output the gradation level after furthersuppressing the input level. The gradation conversion unit 31 may beconfigured, for example, with a lookup table (LUT) or a multiplicationcircuit adapted to multiply the input image data by a given coefficient.If the latter is used, the multiplication coefficient is switched to oneof ka=1.0, ka=0.9 and ka=0.8 correspondingly with the control signalfrom the microcomputer 36. This causes the input image data to bemultiplied by the multiplication coefficient, thus realizing threegradation conversion characteristics shown in FIG. 16, namely, thecharacteristics a to c.

When judging that the screen position to display the image data belongsto the region D₃₁ of the display screen, the microcomputer 36 outputs acontrol signal to the gradation conversion unit 31 to select thegradation conversion characteristic a. That is, the gradation conversioncharacteristic a is selected for the image data to be displayed in theregion D₃₁ of the display screen. Therefore, the image data is output asis (without any conversion) to the LCD control portion 32. On the otherhand, when judging that the screen position to display the image databelongs to the region D₃₂ of the display screen, the microcomputer 36outputs a control signal to the gradation conversion unit 31 to selectthe gradation conversion characteristic b.

That is, the gradation conversion characteristic b is selected for theimage data to be displayed in the region D₃₂ of the display screen.Therefore, the image data is subjected to a gradation conversionprocess. As a result, the display brightness is slightly reduced in theregion D₃₂ of the display screen. Further, when judging that the screenposition to display the image data belongs to the region D₃₃ of thedisplay screen, the microcomputer 36 outputs a control signal to thegradation conversion unit 31 to select the gradation conversioncharacteristic c. That is, the gradation conversion characteristic c isselected for the image data to be displayed in the region D₃₃ of thedisplay screen. Therefore, the image data is subjected to a gradationconversion process. As a result, the display brightness is furtherreduced in the region D₃₃ of the display screen.

This leads to a reduced amount of transmitted light passing through theliquid crystal panel 33 located at the high-voltage side of the linearfluorescent lamps 37 (reduced display brightness), thus realizing aneven brightness distribution over the entire display screen. Asdescribed above, the present embodiment controls the gradation level ofthe image data correspondingly with the screen position to display theimage data, thus reducing the uneven brightness in the longitudinaldirection of the linear fluorescent lamps 37 and ensuring an evenbrightness distribution.

It is to be noted that while the display screen is divided into thethree regions D₃₁ to D₃₃ correspondingly with the longitudinal positionof the linear fluorescent lamps 37 in the above embodiment so that thegradation conversion characteristics a to c are selected for the imagedata displayed respectively in the regions D₃₁ to D₃₃, it is needless tosay that the number of divisions of the display screen and the positionsat which to divide the screen can be changed as appropriatecorrespondingly with the brightness distribution (uneven brightness) inthe longitudinal direction of the linear fluorescent lamps 37.

On the other hand, the reference gradation voltage to drive the liquidcrystal panel may be varied correspondingly with the display screenposition of the liquid crystal panel to compensate for the unevenbrightness in the longitudinal direction of the linear fluorescentlamps.

Embodiment 11

In the liquid crystal display device, the aperture ratio can bealternatively changed correspondingly with the display screen positionof the liquid crystal panel to compensate for the uneven brightness inthe longitudinal direction of the fluorescent lamps and provide adisplay screen with an even brightness. That is, the aperture ratio ofthe liquid crystal panel can be changed correspondingly with thelongitudinal position of the linear fluorescent lamps to reduce theuneven brightness between the two ends of the light source of the linearfluorescent lamps.

In the case of the direct type, for example, the portion of the liquidcrystal panel facing the high-voltage side of the linear fluorescentlamps is formed to have a small aperture ratio so as to reduce theamount of transmitted light passing through the panel, whereas theportion of the liquid crystal panel facing the low-voltage side of thelinear fluorescent lamps is formed to have a large aperture ratio so asto increase the amount of transmitted light passing through the panel.This allows reduction of the uneven brightness in the longitudinaldirection of the linear fluorescent lamps, thus ensuring an evenbrightness distribution. In the case of the edge light type, on theother hand, the aperture ratio of the liquid crystal panel is controlledcorrespondingly with the uneven brightness of the illumination lightacross the surface resulting from the uneven brightness in thelongitudinal direction of the fluorescent lamp. This ensures an evenbrightness distribution.

FIG. 17 illustrates an example of a configuration adapted to control theaperture ratio. In the figure, reference numeral 21 denotes a liquidcrystal panel, 41 a screening film, 42 clear electrodes, 43 TFT driveelements, ian incident light on the liquid crystal panel and o anoutgoing light from the liquid crystal panel. In the liquid crystalpanel 21, the screening film 41 is generally provided that is made of agrid-patterned metal film. In an example of the present embodiment, theaperture ratio of each of the pixels is controlled by the screening film41 correspondingly with the uneven brightness of the fluorescent lampsduring the formation of this film 41. This compensates for the unevenbrightness in the longitudinal direction of the fluorescent lamps withthe pixel-by-pixel optical transmittance, thus achieving a displayscreen with an even brightness.

As is apparent from the above description, the present invention impartsto the backlight unit the brightness compensation means adapted tocompensate for the uneven brightness of the fluorescent lamps so as tocompensate for the uneven brightness in the longitudinal directioninherently present in the linear fluorescent lamps and achieve a displayscreen with an even brightness. This compensates for the difference inbrightness between the high- and low-voltage sides of the fluorescentlamps provided as the light source, thus achieving a backlight unitwhose outgoing light delivers an even brightness. On the other hand,this backlight unit can be used to obtain a liquid crystal displaydevice that delivers an even brightness over the entire display screen.Further, a liquid crystal display device can be obtained that deliversan even brightness over the entire display screen when the image datasupplied to the liquid crystal panel or the aperture ratio of this panelis controlled to compensate for the brightness in the longitudinaldirection of the fluorescent lamps.

1. A backlight unit operable to illuminate the target with fluorescentlamps, the backlight unit comprising brightness compensation meansadapted to compensate for uneven brightness in the longitudinaldirection of the fluorescent lamps.
 2. The backlight unit of claim 1comprising a reflection portion adapted to emit the light from thefluorescent lamps in a specific direction, wherein the brightnesscompensation means are provided on the reflection unit and control thereflectance of the reflection portion to compensate for unevenbrightness in the longitudinal direction of the fluorescent lamps. 3.The backlight unit of claim 2, wherein the brightness compensation meanshave regions with relatively high and low reflectances in the reflectionportion and take advantage of the difference in reflectance tocompensate for uneven brightness in the longitudinal direction of thefluorescent lamps.
 4. The backlight unit of claim 3, wherein thebrightness compensation means have a reflectance gradient that causesthe reflectance of the reflection portion to decline gradually or instages and take advantage of the reflectance gradient to reduce thebrightness of the portion with a relatively high brightness in thelongitudinal direction of the fluorescent lamps.
 5. The backlight unitof claim 3 or 4, wherein the brightness compensation means have areflectance gradient that causes the reflectance of the reflectionportion to increase gradually or in stages and take advantage of thereflectance gradient to increase the brightness of the portion with arelatively low brightness in the longitudinal direction of thefluorescent lamps.
 6. The backlight unit of any one of claims 2 to 5,wherein the brightness compensation means are a dot pattern provided onthe reflection portion and take advantage of the dot pattern to controlthe reflectance of the reflection portion.
 7. The backlight unit ofclaim 6, wherein the reflectance of the reflection portion provided withthe dot pattern is controlled by one or a plurality of the reflectanceof the group of small dots making up the dot pattern, the dot density,the dot shape, and the dot color.
 8. The backlight unit of claim 1comprising a reflection portion adapted to emit the light from thefluorescent lamps in a specific direction, wherein the reflectionportion is made up of first and second reflection layers having givenoptical reflectance and transmittance levels, wherein the reflectionportion is configured with a first region having the first and secondreflection layers stacked one above another in the direction ofincidence of light and a second region made up only of the firstreflection layer, and wherein the reflectance of the reflection portionis controlled using the first region with a relatively high reflectanceand the second region with a reflectance lower than that of the firstregion.
 9. The backlight unit of claim 1, wherein the brightnesscompensation means are provided on a glass tube of the fluorescent lampsand control the transmittance of the glass tube to compensate for unevenbrightness in the longitudinal direction of the fluorescent lamps. 10.The backlight unit of claim 1 comprising a diffusion portion adapted todiffuse the light from the fluorescent lamps, wherein the brightnesscompensation means are provided on the diffusion portion and control thetransmittance of the diffusion portion to compensate for unevenbrightness in the longitudinal direction of the fluorescent lamps. 11.The backlight unit of claim 9 or 10, wherein the brightness compensationmeans have regions with relatively high and low transmittances in theglass tube or the diffusion portion and take advantage of the differencein the transmittance to compensate for uneven brightness in thelongitudinal direction of the fluorescent lamps.
 12. The backlight unitof claim 11, wherein the brightness compensation means have atransmittance gradient that causes the transmittance to declinegradually or in stages and take advantage of the transmittance gradientto reduce the brightness of the portion with a relatively highbrightness in the longitudinal direction of the fluorescent lamps. 13.The backlight unit of claim 11 or 12, wherein the brightnesscompensation means have a transmittance gradient that causes thetransmittance to increase gradually or in stages and take advantage ofthe transmittance gradient to increase the brightness of the portionwith a relatively low brightness in the longitudinal direction of thefluorescent lamps.
 14. The backlight unit of any one of claims 9 to 13,wherein the brightness compensation means are a dot pattern provided onthe glass tube of the fluorescent lamps or the diffusion portion andtake advantage of the dot pattern to control the transmittance.
 15. Thebacklight unit of claim 14, wherein the transmittance of the glass tubeor the diffusion portion provided with the dot pattern is controlled byone or a plurality of the reflectance of the group of small dots makingup the dot pattern, the dot density, the dot shape, and the dot color.16. The backlight unit of claim 1, wherein the brightness compensationmeans are provided on the glass tube of the fluorescent lamps andcontrol the tube surface brightness of the glass tube to compensate foruneven brightness in the longitudinal direction of the fluorescentlamps.
 17. The backlight unit of claim 16, wherein the thickness of thefluorescent substance formed inside the glass tube of the fluorescentlamps as the brightness compensation means is changed correspondinglywith the longitudinal position of the fluorescent lamps to compensatefor uneven brightness in the longitudinal direction of the fluorescentlamps.
 18. A liquid crystal display device comprising the backlight unitof any one of claims 1 to 17 and a liquid crystal panel illuminated bythe backlight unit.
 19. A liquid crystal display device operable toapply an illumination light from a backlight unit having fluorescentlamps to a liquid crystal panel to display images, the liquid crystaldisplay device comprising brightness compensation means adapted tocompensate for uneven brightness in the longitudinal direction of thefluorescent lamps.
 20. The liquid crystal display device of claim 19,wherein the brightness compensation means have a gradation conversionportion operable to carry out a given gradation conversion process ofinput image data and a control portion operable to switch betweengradation conversion characteristics of the gradation conversion portionbased on a synchronizing signal of the input image data, and wherein thecontrol portion switches from one gradation conversion characteristic toanother in the gradation conversion portion based on the screen positionto display the image data to compensate for uneven brightness in thelongitudinal direction of the fluorescent lamps.
 21. The liquid crystaldisplay device of claim 19, wherein the liquid crystal panel isconfigured to have, as the brightness compensation means, an apertureratio that changes correspondingly with the display screen position, andwherein the aperture ratio is changed to compensate for unevenbrightness in the longitudinal direction of the fluorescent lamps.